The Earth gravity field can be measured with high precision by constructing the purely gravitational orbit of the inner-satellite in Inner- formation Flying System (IFS), which offers a new way to carry out gravity satellite measurement without accelerometers. Since it is difficult to analytically evaluate gravity field measurement performance by IFS, numerical simulation for gravity field measurement was done based on MATLAB parallel programs, and the highest degree of recovered gravity field model and the corresponding geoid and gravity anomaly errors were obtained. Under the condition that IFS orbit altitude is 300km, non gravitational interference on the inner-satellite is 1.0×10^-10m/s², the outer-satellite orbit determination error is 3cm and the measurement error of relative position between the inner- satellite and outer-satellite is 1mm, the highest degree of recovered gravity field model is 72, the corresponding geoid cumulative error is 44cm, and the corresponding gravity anomaly cumulative error is 4.5mGal. It is known that the effective frequency band of gravity field measurement by IFS is at the lower degree.